Not applicable.
1. Field of the Invention
The present invention relates generally to systems and methods for increasing the power produced by a gas turbine or combustion turbine for driving a mechanical device or for power generation. More particularly, it provides a more efficient refrigeration method and apparatus for cooling turbine inlet air to enhance its power output and overall combustion efficiency.
2. Background of the Invention
As used herein, the terms turbine, gas turbine and combustion turbine may be used interchangeably in reference to the same or similar process or system. Gas turbines are widely used in all phases of industrial applications. They are utilized as a source of shaft power to drive compressors, aircraft, and other rotating equipment. They are also coupled to electrical power generators for the generation of electricity extensively in either a simple cycle or a combined cycle power plant. Gas turbines typically consist of an intake air filtration, a compressor for compressing inlet air, a combustion chamber for mixing and igniting the compressed air with fuel to form a compressed hot gas for expansion to a turbine section to generate power. The work extracted from the high temperature gas, after partially used for air compression, will be available for output load. The exhaust gas from the turbine section, which contains a high level of heat energy, can be introduced into a waste heat recovery section, e.g. the heat recovery steam generator (HGSG) in a combined cycle power plant, or in some cases, discarded.
The performance of a combustion turbine system operated under the cycle described above is generally proportional to the mass flow rate of the inlet air to the gas turbine compressor, and is therefore largely affected by ambient air conditions. At high ambient temperatures, the available work produced from a gas turbine decreases due to a reduction in the mass flow of air through the system. And ironically, power demand often reaches the peak in most gas turbine applications during the hottest days when the operational efficiency of the turbine is at the lowest. Thus, an inlet air cooling system is commonly adopted to reduce the intake air temperature for minimizing the impact on turbine output, and to augment power output even during hot days when it can be installed cost effectively.
Various methods and apparatus for cooling gas-turbine inlet air are available in the art. For example, U.S. Pat. No. 5,930,990 to Zachary, et al. discloses an apparatus for achieving power augmentation in a gas turbine through a wet compression where water is sprayed to the inlet air to induce xe2x80x9clatent heat inter-cooling.xe2x80x9d Further, a liquid coolant fuel, as exemplified by the disclosure in U.S. Pat. No. 5,806,298, is introduced at the inlet of the air compressor, which vaporizes and cools the air to enhance power output of a gas turbine. Others utilize either a direct or an indirect evaporative cooler where the heat of hot air is transferred into the circulating water, leading to partial vaporization of water. However, the temperature reduction achieved with an evaporative cooler is limited to the daily fluctuating wet bulb temperatures in the areas. An evaporative cooling apparatus may not be applicable for warm and humid areas. Moreover, it often requires a high level of maintenance and relies on the quality and availability of a water source.
It is also readily common to introduce an external refrigeration system to chill the inlet air temperature far below that achievable by an evaporative cooler. This approach permits the turbine to operate at a fairly constant and optimal output regardless of the ambient air conditions. Although chilling the air to near 32xc2x0 F. is possible, a minimum temperature considered suitable for inlet air chilling in a gas turbine application is usually set above 42xc2x0 F. This prevents moisture contained in the inlet air from freezing and depositing on the inlet guide vanes or compressor blades as the static air temperature decreases further while it accelerates into the compression chamber. U.S. Pat. No. 5,457,951 discloses the use of liquefied natural gas as a refrigerant to improve the capacity and efficiency of a combined cycle power plant. Liquid nitrogen, as disclosed in U.S. Pat. No. 5,697,207, was also proposed to gain additional power from a gas turbine generator. However, the availability of this type of cold refrigerant is extremely limited. In most areas where a cold refrigerant is not readily available, a refrigeration system is proposed.
In all refrigeration systems, the refrigeration process depends on the absorption of heat at a low temperature which is achieved by the expansion and evaporation of a liquid refrigerant. Refrigeration systems are distinguished by how the refrigerant vapor is liquefied to repeat the cycle. There are two major types of refrigeration systems in commercial practice today, namely absorption refrigeration and mechanical refrigeration. In a typical absorption refrigeration system, a refrigerant vapor from the evaporator is dissolved in a liquid absorbent to form what is commonly referred to as a xe2x80x9csolution pairxe2x80x9d in an absorber. The solution pair is transferred to a desorber, or regenerator, where heat energy is applied to desorb the refrigerant in the form of a vapor, which is fed to a condenser. The two most commonly used absorption refrigeration systems are ammonia water and aqueous lithium bromide units. U.S. Pat. No. 5,555,738 improves combined-cycle power plant efficiency by operating an ammonia refrigeration cycle driven by the waste heat from the gas turbine to lower the inlet air temperature. Although absorption refrigeration systems are known and utilized commercially, continuous efforts have been devoted to improving their performance. A multiple effect generator is described in U.S. Pat. Nos. 4,183,228; 4,742,693, and 4,441,3332 to improve the efficiency of an absorption refrigeration circuit. U.S. Pat. Nos. 4,283,918 and 4,413,479 introduce a third fluid, which is at least partially immiscible to allow separation of refrigerant at absorption temperature, in the absorption refrigeration cycle. Other improvements include those described in U.S. Pat. Nos. 4,055,964 and 5,816,070. These systems are driven by heat energy and are relatively inefficient and inflexible unless reliable waste heat or inexpensive fuels are readily available.
In a mechanical refrigeration system, the refrigerant vapor is mechanically compressed to a high pressure and is then cooled to total condensation. This type of system has prevailed in industrial installations as a result of the improvement in efficiency. Depending upon temperature requirements, availability, and economics, various pure component refrigerants are commercially available, including light hydrocarbons, ammonia, water, and newly discovered chlorinated fluorocarbons (CFC""s). For instance, an inlet air chilling apparatus using water vapor compression is described in U.S. Pat. No. 5,632,148 to achieve power augmentation of a gas turbine. For the modest cooling goal of inlet air chilling, the CFC refrigerants may be most appealing. However, their usage has become increasingly restricted due to environmental regulations. Conventional mechanical refrigeration using a single component refrigerant capable of achieving much colder refrigeration tends to be less efficient. Besides, the need of additional power to drive the compressor reduces the advantages of inlet air chilling.
An enhanced refrigeration system has also been attempted by combining both mechanical refrigeration and absorption refrigeration. For instance, U.S. Pat. No. 5,038,572 discloses a combined refrigeration method and apparatus for an improved efficiency, wherein mechanical refrigeration is alternately connected in series with an aqueous lithium bromide refrigeration. A combustion-powered compound refrigeration system is disclosed in U.S. Pat. No. 4,873,839 to reduce the energy consumption of a refrigeration system wherein the hot exhaust gas from a combustion engine, used to power the refrigerant compressor, is utilized to drive an ammonia absorption unit. U.S. Pat. No. 4,586,344 to Lutz, et al., incorporated herein by reference, introduces a pair of refrigerants which form a substantially immiscible fluid having a total pressure substantially greater than the vapor pressure of either individual refrigerant in the evaporative chiller. This process leads to a higher suction pressure and lower compression horsepower for a mechanical refrigeration system. U.S. Pat. No. 5,816,070 to Mechler teaches the use of vapor recompression absorption to increase the efficiency of an absorption process.
Still others, such as U.S. Pat. Nos. 5,353,597; 5,537,813; and 6,119,445, propose to increase inlet air density by a combination of inlet air compression and cooling.
As can be seen from the foregoing description, prior art has long sought methods for improving operational capacity and efficiency of a gas turbine, particularly in hot weather conditions. While inlet air chilling appears to offer the most advantages, there continues to be a need for improved methods and apparatus to lower costs and energy consumption associated with the provision of such a system.
It is an object of the present invention to provide a more efficient and economical refrigeration system to augment the power output of a gas turbine. A significant reduction in the power required to drive the refrigerant compressor can be achieved by the addition of an absorptive refrigerant to the evaporative chiller, wherein a substantial increase in pressure results from the combined refrigerant. The absorptive refrigerant vapor from the chiller is subsequently separated from the mechanical refrigerant in an absorber by adding a liquid absorbent, which absorbs the absorptive refrigerant over the mechanical refrigerant.
It is another object of the present invention to reduce the usage of the combustion fuel by utilizing the hot exhaust gas from the gas turbine for the generation of the absorptive refrigerant. Consequently, the emissions of greenhouse gases resulting from the integrated inlet air chilling system can be reduced.
In carrying out these and other objects of the invention, there is provided, in the broadest sense, an inlet air chiller using a combined refrigerant to increase inlet air density for optimizing the performance of a combustion turbine system. The hybrid refrigeration system is based on a combination of mechanical refrigeration supplemented by an absorption refrigeration cycle to reduce the compression requirements over a conventional refrigeration system using a single component refrigerant. At least two refrigerants, a mechanical refrigerant and an absorptive refrigerant, are utilized in the evaporative chiller wherein the combined refrigerant exhibits the characteristic of a much higher total pressure than the vapor pressure of each individual refrigerant at the refrigeration temperature regardless of their miscibility. Preferably, the system includes two substantially immiscible refrigerants which coexist where the total system pressure, in most cases, is approximately equivalent to the sum of the vapor pressures of each refrigerant. This can be exemplified below by a binary propane-ammonia system where experimental vapor pressures representative of such systems were published in The Journal of Chemical and Engineering Data, by Noda et al., entitled xe2x80x9cIsothermal Vapor-Liquid and Liquid-Liquid Equilibria for the Propane-Ammonia and Propylene-Ammonia Systems.xe2x80x9d
As shown, the vapor pressure of the two co-existing liquid phases (ammonia and propane) is 129.4 psia at 32xc2x0 F., which is almost double the vapor pressure of each individual pure refrigerant, namely 68.6 psia for propane and 62.4 psia for ammonia. The compression power needed for the refrigerant compressor is greatly reduced due to a higher suction pressure of the resultant refrigerant vapor from the chiller.
In the present invention, the resultant combined refrigerant from the evaporator is preferably preheated to a temperature well above water freezing temperature and then directly fed to an absorber wherein the absorptive refrigerant is separated from the mechanical refrigerant by the addition of a liquid absorbent. The mechanical refrigerant vapor, essentially not soluble in the liquid absorbent, from the absorber is compressed and subsequently condensed. The absorptive refrigerant is heat regenerated from a solution pair in the desorber. By removing one of the refrigerants as in the present invention prior to mechanical compression, the mass flow into the refrigerant compressor, and thereby power requirements, are further reduced. It should be noted that, in some cases, the vaporized combined refrigerant could be compressed to a higher pressure prior to its introduction into the absorber.
The economic advantages of the present invention are further enhanced by thermally linking the heat required to generate the absorptive refrigerant from the solution pair with the hot exhaust heat available from the gas turbine or the refrigerant compressor driver, if available. This is of significant importance when the cost of combustion fuel is expensive and/or the reduction in greenhouse gases emissions is desired.
The operational efficiency can be further improved in another embodiment of the present invention by applying an economizer to the mechanical refrigerant after the expansion of the mechanical refrigerant. The economizer, operated at an intermediate pressure, permits a portion of the flashed refrigerant vapor to be collected and fed to the refrigerant compressor, thus reducing the flow to the chiller and absorber.